The invention relates to a pressable glass ceramic, and in particular to a low-melting, high-expansion glass ceramic. Further, the invention relates to a method for the fabrication of such glass ceramic, and a method for the fabrication of a dental restoration using such glass ceramic. Finally, the invention relates to a component which makes it possible to increase the coefficients of thermal expansion (CTEs) of glass compositions, specifically in such a manner that the glass compositions, also after some temperature treatments, maintain a stable CTE.
In more detail, the invention relates to porcelains or ceramics of lithium disilicate glass which are plastically deformable under the influence of heat and pressure.
With the glass ceramic according to the invention, for instance all-ceramic dental restorations, inlays and onlays can be fabricated, but it can also be applied for carrying out repair on existing restorations or other types of dental prostheses and in combination with metal alloys.
EP-A-0 827 941 discloses a sinterable lithium disilicate glass ceramic which has a CTE of 9-10 xcexcm/m.K (measured in the temperature range of 20-500xc2x0 C.). This is too low to allow firing on with conventional porcelain masses which are used for noble metal alloys. Since dental technicians most preferably work with porcelains that can be processed in an approximately equal manner and which possess substantially equal, at least compatible properties, it would be desirable to increase this relatively low CTE, in any case to above 12 xcexcm/m.K (measured in the range of 20-500xc2x0 C.).
More particularly, the press ceramic constituting the invention of EP-A-0 827 941, while having a high strength of fracture, a low pressing temperature and a reasonable translucency, has the disadvantage that the expansion is too low to allow firing on with current fire-on ceramics which for the greater part possess a higher coefficient of thermal expansion.
One of the options immediately eligible for this purpose is the addition of a leucite-containing high-expansion glass frit, since leucite, as is well known, raises the CTE. However, when known leucite-containing high-expansion glass frits, for instance Component 1 as described by Weinstein in his trend-setting U.S. patent U.S. Pat. No. 3,052,982 or high-expansion frits as described in EP-A-0 475 528, are added to the just described lithium disilicate glass according to EP-A-0 827 941, the proportion of aluminum oxide present in this high-expansion glass frit reacts with lithium silicate to form aluminum-lithium silicate, which exhibits a CTE around zero. Accordingly, instead of a CTE-increasing effect, the application of such known leucite-containing high-expansion frits has a lowering effect on the CTE value, so that the objective is not achieved. Further, investigations of the present inventor have shown that if after addition of high-expansion leucite-containing glass the lithium silicate material initially exhibits an increased expansion, further reactions between the leucite glass frit and the lithium silicate cannot be prevented when that material undergoes conventional processing heat treatments. This leads to the situation where during the different fire-on phases of the porcelain, a progressively decreasing CTE is found.
U.S. Pat. No. 6,120,591 discloses a dental porcelain which consists of a glassy matrix with crystals of tetragonal leucite embedded therein. This porcelain has a maturing temperature of 600-885xc2x0 C. and a CTE of 11-19 xcexcm/m.K (measured in the temperature range of from 25 to 500xc2x0 C.). The tetragonal leucite is preferably fine-grained, and preferably has a diameter of 1-3 xcexcm.
This tetragonal leucite is formed by mixing powdered metal oxides and metal carbonates in the appropriate proportions, whereafter the mixed powders are heated until a glass melt forms. This melt is quenched, whereafter the glass is heated to an elevated temperature of 950-1100xc2x0 C. and thus held for 1-6 hours, whereby crystalline material is formed and grows further. Optionally, the quench step can be omitted.